To provide a filling body manufacturing method capable of simultaneously performing a step of a filling fluid in an interior bag and a step of erecting an exterior box, a filling body and a fluid packaging container, and a filling body capable of easily preventing flow-in of air into the interior bag under the filling operation and reducing the effect on the content on the sanitary aspect.

In the filling body manufacturing method having a paper-made exterior box with a flap to constitute at least one of a top side and a bottom side continuously connected to upper and lower sides of at least four side surfaces connected in a ring via a folding line, a plastic inner bag affixed to at least one side surface of the exterior box, and the fluid filled in the interior bag, the fluid is filled in the interior bag, the exterior box is erected by the filling pressure of the interior bag, the interior bag is then closed, and the flap is folded by the folding line to assemble the exterior box.